Spurious warning on configuring `nosignatures`

Using something like this:

```rst
.. automodsumm:: MODULE
   :nosignatures:
```

Works as intended (signatures are not included), but produces this spurious warning:

```
.venv/lib/python3.8/site-packages/sphinx_automodapi/utils.py:179: UserWarning:    :nosignatures:
  warn(line)
```

https://github.com/astropy/sphinx-automodapi/blob/main/sphinx_automodapi/utils.py#L178-L180